A gas boiler for producing hot water normally comprises a gas burner, and at least one heat exchanger through which combustion fumes and water flow. Some types of gas boilers, known as condensation boilers, condense the steam in the combustion fumes and transfer the latent heat in the fumes to the water. Condensation boilers are further divided into a first type, equipped with a first exchanger close to the burner, and a second exchanger for simply condensing the fumes; and a second type, equipped with only one heat exchanger which provides solely for thermal exchange along a first portion, and for both thermal exchange and fume condensation along a second portion.
A condensation or dual-function exchanger of the above type is disclosed in WO 2004/090434 and comprises a casing extending along a first axis and through which combustion fumes flow; a tube along which water flows, and which is housed inside said casing and coils about the first axis to form a helix comprising a succession of turns; and deflecting means for directing the fumes between successive turns in a first direction perpendicular to said first axis. The tube is finned with at least a first and second outward fins facing one another and extending along the length of the tube.
Even though the above identified heat exchanger proved to be extremely effective in term of heat exchange, has still the drawback that the distance between the first and second outward fins of adjacent turns cannot be freely selected to optimise the heat exchange because the convexity of the tube protruding from the outward fins imposes a limit to such a distance to let the fumes flow with an adequate speed.